make netinet/in.h suppress clashing definitions from kernel headers

make netinet/in.h suppress clashing definitions from kernel headers

the linux kernel uapi headers provide their own definitions of the
structures from netinet/in.h, resulting in errors when a program
includes both the standard libc header and one or more of the
networking-related kernel headers that pull in the kernel definitions.

as before, we do not attempt to support the case where kernel headers
are included before the libc ones, since the kernel definitions may
have subtly incorrect types, namespace violations, etc. however, we
can easily support the inclusion of the kernel headers after the libc
ones, since the kernel headers provide a public interface for
suppressing their definitions. this patch adds the necessary macro
definitions for such suppression.